244 Transactions of the Royal Society of South Africa. 
molecules are anisotropic unities ; that their arrangement in the naturally 
occurring forms may be due to strains, but that their crystallographic 
orientation, on which the sign of the double refraction depends, is due to 
their water content. 
One of us * has shown in a previous paper how, by gradually driving off 
the water which it contains, the internal molecular structure of brucite is 
changed ; this change is tantamount to an alteration of the axial ratio of 
the mineral. Keeping this in mind, and taking into consideration also the 
fact that whenever the clear negative hyalite is heated to redness it becomes 
converted into an opalescent positive material resembling the naturally 
occurring opalescent hyalite, we cannot help but feel that the water content 
is the most important factor in deciding the nature of the double refraction 
of hyalite. 
That tangential sections of hyalite behave as uniaxial — or biaxial ? — 
mineral substances we regard as evidence in support of our contention that 
the molecules are in themselves anisotropic. 
In conclusion, we wish to thank the Royal Society of South Africa for 
the grant in aid of research. 
* J. S. v. d. Lingen, Trans. R. Soc. S. Afric, Vol. VII, Pt. 1, 1918, p. 59. 
